1. Field of the Invention
The present invention relates to a video special effect system for obtaining superimposed color television pictures from the addition of two picture signals or two picture signal groups supplied by two video sources such as video cameras or video tape recorders.
2. Description of the Prior Art
Hereto, superimposed image effects are obtained in black and white television as well as in color television by means of two variable gain amplifiers respectively receiving two picture signals and having completely independent gain controls, the latter usually consisting of adjustable resistors, for example, of two potentiometers. An adder circuit mixes the two amplified picture signals into a resultant picture signal.
Separate adjustment of the potentiometers entails the danger that the resulting signal, resulting from the combination of the two amplified picture signals, exceeds a standard peak amplitude. Such overshooting requires peak limiting or chopping or the resulting signal and, accordingly, leads to distortion of the picture produced by the resulting signal, such as loss of half-tones in the background scene picture on which is superimposed the other, or foreground scene picture representing for instance a ghost, a veil or a rain curtain, or loss of details in the background scene. Such picture defects are more pronounced if the amplitude of the signal representative of either of the two pictures approaches a reference white level corresponding to maximum amplitude. On the other hand, when the amplitude of the signal corresponding to either picture is near the black level, noise and other disturbances, such as spots, affect the resulting signal picture.